Engine Configuration¶

The Engine is the starting point for any SQLAlchemy application. It’s “home base” for the actual database and its DBAPI, delivered to the SQLAlchemy application through a connection pool and a Dialect, which describes how to talk to a specific kind of database/DBAPI combination.

The general structure can be illustrated as follows:

Where above, an Engine references both a Dialect and a Pool, which together interpret the DBAPI’s module functions as well as the behavior of the database.

Creating an engine is just a matter of issuing a single call, create_engine():

from sqlalchemy import create_engine
engine = create_engine('postgresql://scott:tiger@localhost:5432/mydatabase')

The above engine creates a Dialect object tailored towards PostgreSQL, as well as a Pool object which will establish a DBAPI connection at localhost:5432 when a connection request is first received. Note that the Engine and its underlying Pool do not establish the first actual DBAPI connection until the Engine.connect() method is called, or an operation which is dependent on this method such as Engine.execute() is invoked. In this way, Engine and Pool can be said to have a lazy initialization behavior.

The Engine, once created, can either be used directly to interact with the database, or can be passed to a Session object to work with the ORM. This section covers the details of configuring an Engine. The next section, Working with Engines and Connections, will detail the usage API of the Engine and similar, typically for non-ORM applications.

Supported Databases¶

SQLAlchemy includes many Dialect implementations for various backends. Dialects for the most common databases are included with SQLAlchemy; a handful of others require an additional install of a separate dialect.

See the section Dialects for information on the various backends available.

Database Urls¶

The create_engine() function produces an Engine object based on a URL. These URLs follow RFC-1738, and usually can include username, password, hostname, database name as well as optional keyword arguments for additional configuration. In some cases a file path is accepted, and in others a “data source name” replaces the “host” and “database” portions. The typical form of a database URL is:

dialect+driver://username:password@host:port/database

Dialect names include the identifying name of the SQLAlchemy dialect, a name such as sqlite, mysql, postgresql, oracle, or mssql. The drivername is the name of the DBAPI to be used to connect to the database using all lowercase letters. If not specified, a “default” DBAPI will be imported if available - this default is typically the most widely known driver available for that backend.

As the URL is like any other URL, special characters such as those that may be used in the password need to be URL encoded. Below is an example of a URL that includes the password "kx%jj5/g":

postgresql+pg8000://dbuser:kx%25jj5%2Fg@pghost10/appdb

The encoding for the above password can be generated using urllib:

>>> import urllib.parse
>>> urllib.parse.quote_plus("kx%jj5/g")
'kx%25jj5%2Fg'

Examples for common connection styles follow below. For a full index of detailed information on all included dialects as well as links to third-party dialects, see Dialects.

PostgreSQL¶

The PostgreSQL dialect uses psycopg2 as the default DBAPI. pg8000 is also available as a pure-Python substitute:

# default
engine = create_engine('postgresql://scott:tiger@localhost/mydatabase')

# psycopg2
engine = create_engine('postgresql+psycopg2://scott:tiger@localhost/mydatabase')

# pg8000
engine = create_engine('postgresql+pg8000://scott:tiger@localhost/mydatabase')

More notes on connecting to PostgreSQL at PostgreSQL.

MySQL¶

The MySQL dialect uses mysql-python as the default DBAPI. There are many MySQL DBAPIs available, including MySQL-connector-python and OurSQL:

# default
engine = create_engine('mysql://scott:tiger@localhost/foo')

# mysqlclient (a maintained fork of MySQL-Python)
engine = create_engine('mysql+mysqldb://scott:tiger@localhost/foo')

# PyMySQL
engine = create_engine('mysql+pymysql://scott:tiger@localhost/foo')

More notes on connecting to MySQL at MySQL.

Oracle¶

The Oracle dialect uses cx_oracle as the default DBAPI:

engine = create_engine('oracle://scott:tiger@127.0.0.1:1521/sidname')

engine = create_engine('oracle+cx_oracle://scott:tiger@tnsname')

More notes on connecting to Oracle at Oracle.

Microsoft SQL Server¶

The SQL Server dialect uses pyodbc as the default DBAPI. pymssql is also available:

# pyodbc
engine = create_engine('mssql+pyodbc://scott:tiger@mydsn')

# pymssql
engine = create_engine('mssql+pymssql://scott:tiger@hostname:port/dbname')

More notes on connecting to SQL Server at Microsoft SQL Server.

SQLite¶

SQLite connects to file-based databases, using the Python built-in module sqlite3 by default.

As SQLite connects to local files, the URL format is slightly different. The “file” portion of the URL is the filename of the database. For a relative file path, this requires three slashes:

# sqlite://<nohostname>/<path>
# where <path> is relative:
engine = create_engine('sqlite:///foo.db')

And for an absolute file path, the three slashes are followed by the absolute path:

# Unix/Mac - 4 initial slashes in total
engine = create_engine('sqlite:////absolute/path/to/foo.db')

# Windows
engine = create_engine('sqlite:///C:\\path\\to\\foo.db')

# Windows alternative using raw string
engine = create_engine(r'sqlite:///C:\path\to\foo.db')

To use a SQLite :memory: database, specify an empty URL:

engine = create_engine('sqlite://')

More notes on connecting to SQLite at SQLite.

Others¶

See Dialects, the top-level page for all additional dialect documentation.

Engine Creation API¶

Object Name	Description
create_engine(args, *kwargs)	Create a new `Engine` instance.
engine_from_config(configuration[, prefix], **kwargs)	Create a new Engine instance using a configuration dictionary.
make_url(name_or_url)	Given a string or unicode instance, produce a new URL instance.
URL	Represent the components of a URL used to connect to a database.

function sqlalchemy.create_engine(*args, **kwargs)¶

Create a new Engine instance.

The standard calling form is to send the URL as the first positional argument, usually a string that indicates database dialect and connection arguments:

engine = create_engine("postgresql://scott:tiger@localhost/test")

Additional keyword arguments may then follow it which establish various options on the resulting Engine and its underlying Dialect and Pool constructs:

engine = create_engine("mysql://scott:tiger@hostname/dbname",
                            encoding='latin1', echo=True)

The string form of the URL is dialect[+driver]://user:password@host/dbname[?key=value..], where dialect is a database name such as mysql, oracle, postgresql, etc., and driver the name of a DBAPI, such as psycopg2, pyodbc, cx_oracle, etc. Alternatively, the URL can be an instance of URL.

**kwargs takes a wide variety of options which are routed towards their appropriate components. Arguments may be specific to the Engine, the underlying Dialect, as well as the Pool. Specific dialects also accept keyword arguments that are unique to that dialect. Here, we describe the parameters that are common to most create_engine() usage.

Once established, the newly resulting Engine will request a connection from the underlying Pool once Engine.connect() is called, or a method which depends on it such as Engine.execute() is invoked. The Pool in turn will establish the first actual DBAPI connection when this request is received. The create_engine() call itself does not establish any actual DBAPI connections directly.

Pooling¶

The Engine will ask the connection pool for a connection when the connect() or execute() methods are called. The default connection pool, QueuePool, will open connections to the database on an as-needed basis. As concurrent statements are executed, QueuePool will grow its pool of connections to a default size of five, and will allow a default “overflow” of ten. Since the Engine is essentially “home base” for the connection pool, it follows that you should keep a single Engine per database established within an application, rather than creating a new one for each connection.

Note

QueuePool is not used by default for SQLite engines. See SQLite for details on SQLite connection pool usage.

For more information on connection pooling, see Connection Pooling.

Custom DBAPI connect() arguments¶

Custom arguments used when issuing the connect() call to the underlying DBAPI may be issued in three distinct ways. String-based arguments can be passed directly from the URL string as query arguments:

db = create_engine('postgresql://scott:tiger@localhost/test?argument1=foo&argument2=bar')

If SQLAlchemy’s database connector is aware of a particular query argument, it may convert its type from string to its proper type.

create_engine() also takes an argument connect_args which is an additional dictionary that will be passed to connect(). This can be used when arguments of a type other than string are required, and SQLAlchemy’s database connector has no type conversion logic present for that parameter:

db = create_engine('postgresql://scott:tiger@localhost/test', connect_args = {'argument1':17, 'argument2':'bar'})

The most customizable connection method of all is to pass a creator argument, which specifies a callable that returns a DBAPI connection:

def connect():
    return psycopg.connect(user='scott', host='localhost')

db = create_engine('postgresql://', creator=connect)

Configuring Logging¶

Python’s standard logging module is used to implement informational and debug log output with SQLAlchemy. This allows SQLAlchemy’s logging to integrate in a standard way with other applications and libraries. There are also two parameters create_engine.echo and create_engine.echo_pool present on create_engine() which allow immediate logging to sys.stdout for the purposes of local development; these parameters ultimately interact with the regular Python loggers described below.

This section assumes familiarity with the above linked logging module. All logging performed by SQLAlchemy exists underneath the sqlalchemy namespace, as used by logging.getLogger('sqlalchemy'). When logging has been configured (i.e. such as via logging.basicConfig()), the general namespace of SA loggers that can be turned on is as follows:

sqlalchemy.engine - controls SQL echoing. set to logging.INFO for SQL query output, logging.DEBUG for query + result set output. These settings are equivalent to echo=True and echo="debug" on create_engine.echo, respectively.
sqlalchemy.pool - controls connection pool logging. set to logging.INFO to log connection invalidation and recycle events; set to logging.DEBUG to additionally log all pool checkins and checkouts. These settings are equivalent to pool_echo=True and pool_echo="debug" on create_engine.echo_pool, respectively.
sqlalchemy.dialects - controls custom logging for SQL dialects, to the extend that logging is used within specific dialects, which is generally minimal.
sqlalchemy.orm - controls logging of various ORM functions to the extent that logging is used within the ORM, which is generally minimal. Set to logging.INFO to log some top-level information on mapper configurations.

For example, to log SQL queries using Python logging instead of the echo=True flag:

import logging

logging.basicConfig()
logging.getLogger('sqlalchemy.engine').setLevel(logging.INFO)

By default, the log level is set to logging.WARN within the entire sqlalchemy namespace so that no log operations occur, even within an application that has logging enabled otherwise.

The echo flags present as keyword arguments to create_engine() and others as well as the echo property on Engine, when set to True, will first attempt to ensure that logging is enabled. Unfortunately, the logging module provides no way of determining if output has already been configured (note we are referring to if a logging configuration has been set up, not just that the logging level is set). For this reason, any echo=True flags will result in a call to logging.basicConfig() using sys.stdout as the destination. It also sets up a default format using the level name, timestamp, and logger name. Note that this configuration has the affect of being configured in addition to any existing logger configurations. Therefore, when using Python logging, ensure all echo flags are set to False at all times, to avoid getting duplicate log lines.

The logger name of instance such as an Engine or Pool defaults to using a truncated hex identifier string. To set this to a specific name, use the “logging_name” and “pool_logging_name” keyword arguments with sqlalchemy.create_engine().

Note

The SQLAlchemy Engine conserves Python function call overhead by only emitting log statements when the current logging level is detected as logging.INFO or logging.DEBUG. It only checks this level when a new connection is procured from the connection pool. Therefore when changing the logging configuration for an already-running application, any Connection that’s currently active, or more commonly a Session object that’s active in a transaction, won’t log any SQL according to the new configuration until a new Connection is procured (in the case of Session, this is after the current transaction ends and a new one begins).

SQLAlchemy 1.2 Documentation

SQLAlchemy 1.2 Documentation

SQLAlchemy Core

Engine Configuration¶

Supported Databases¶

Database Urls¶

PostgreSQL¶

MySQL¶

Oracle¶

Microsoft SQL Server¶

SQLite¶

Others¶

Engine Creation API¶

Pooling¶

Custom DBAPI connect() arguments¶

Configuring Logging¶